1. Technical Field
The present invention pertains to optical communication systems. In particular, the present invention pertains to a multimode fiber optic amplifier employing dynamic gain adjustment to produce amplified optical signals with reduced noise and at desired levels to prevent saturation of an optical detector. The present invention amplifier is preferably utilized within a receiving unit for an optical communications system.
2. Discussion of Related Art
Optical communication systems transmit information in the form of optical signals through the environment between optical communication units. The transmitted signals typically encounter atmospheric and other conditions. An optical communication unit generally employs a large optic to focus received optical signals directly onto a detector for signal detection. However, some communications units may utilize an amplifier to amplify received optical signals and enable detection of weaker signals.
Optical amplifiers for amplifying optical signals within optical communication systems have generally been implemented based on single mode fibers. These types of amplifiers tend to have reduced noise. Basically, an optical fiber is typically cylindrical and includes a central portion or core surrounded by an optical material or cladding. Light or optical signals are guided by the fiber through the core, while the cladding maintains the light within the core by internal reflection. Single mode fibers have a core with small dimensions, thereby enabling light to traverse the core in a single ray. In contrast, multimode fibers include a core with greater dimensions enabling light to traverse the core in a plurality of rays or modes.
However, single mode amplifiers suffer from several disadvantages. Initially, the probability of reception of a transmitted optical signal at an optical communication unit is enhanced in accordance with the diameter and field of view (e.g., commonly referred to as the numerical aperture or acceptance angle) of an optical fiber receiving the transmitted signal. These fiber characteristics should be maximized for a particular application to achieve increased reception probability. Since single mode fibers generally have a relatively small diameter and a standard numerical aperture, the probability of reception of a transmitted signal via a single mode fiber is limited. Further, single mode fibers require complex connections relative to multimode fibers, thereby complicating the amplifier or system.
Although multimode fibers include a greater diameter to enhance reception probability, the additional modes provided by this type of fiber produce noise (e.g., Amplified Spontaneous Emission (ASE)) that may dominate the desired single mode signal.